The present invention relates to methods and apparatus for mixing a gas with a liquid polymeric material to produce a closed cell foam structure such as a foam coating or bead. More particularly, the invention is suitable for use with polymeric materials such as adhesives, sealants and caulks.
The types and quality of foamed polymeric material produced by the methods and apparatus of the present invention are generally similar to those produced by apparatus of the type disclosed in U.S. Pat. No. 4,778,631 (the '631 patent) issued to Cobbs, Jr. et al. and assigned to the assignee of the present invention. The apparatus disclosed in the '631 patent is a dynamic mixer which was developed to address the very difficult problem of homogeneously mixing two different materials having very divergent viscosities or, in other words, having an extremely high viscosity ratio. More specifically, it deals with the problem of homogeneously mixing a gas which essentially has a viscosity near zero ( 1.2.times.10.sup.-6 lbs./ft.-sec.) with liquid polymers having viscosities ranging from about 2000 centipoise (cps) up to, for example, 1,000,000 cps to produce high quality closed cell foam. Although the dynamic mixer of the '631 patent also works very well with polymers having lower ranges of viscosities, it is disclosed as being especially useful and advantageous for mixing gas with liquid polymers having viscosities in the range of 50,000 cps to above 1,000,000 cps.
The dynamic mixer of the '631 patent produces a very high quality closed cell foam such as foamed plastisol which may be used to form a gasket or as a coating such as an auto body undercoating or as a bead for other sealing purposes. The "high quality" of the closed cell foam produced by the dynamic mixer of the '631 patent is characterized by the homogeneous dispersion of microbubbles of gas which remain trapped within the polymeric material after it has been dispensed from the mixer and has cured or set. Prior to the introduction of dynamic mixers of the type disclosed in the '631 patent, apparatus incorporating gear pumps had been utilized to foam so-called "hot melt" adhesives which range in viscosity from about 2,200 cps to 20,000-35,000 cps at the usual dispensing temperatures of about 350.degree. F. to 400.degree. F. One example of this type of apparatus is disclosed in U.S. Pat. No. 4,059,714 to Scholl et al.
The '631 patent addressed some of the shortcomings of gear pump mixers, especially with respect to the inability to obtain adequate mixing with polymers having viscosities above 50,000 cps, however, both gear pump mixers and dynamic mixers of the type disclosed in the '631 patent have significant drawbacks related to their cost effectiveness and practicality in certain applications. Most notably, these mixers are relatively bulky and complex in design due to their large number of precision moving parts. Therefore, they may be relatively difficult to set up especially where space considerations are a significant factor and, perhaps more importantly, are expensive to manufacture and therefore costly to the end user. Also, the cost of maintenance and repair remains high throughout the life of the mixers due to their relatively complex design.
Two other mixing devices designed to mix liquid polymers with gas are disclosed in U.S. Pat. No. 4,396,529 (the '529 patent) to Price et al. and U.S. Pat. No. 4,527,712 (the '712 patent) to Cobbs, Jr. et al., both being assigned to the assignee of the present invention. Like the above-mentioned patent to Scholl et al., the '529 patent concerns the foaming of hot melt liquid adhesives. Specifically, the '529 patent discloses a dispensing head including a static mixing means preferably comprising four baffle plates disposed directly upstream of the dispensing head discharge orifice. Pressurized gas is injected into a contact chamber containing pressurized liquid hot melt adhesive immediately upstream of the four baffle plates. As the liquid hot melt adhesive and gas are caused to flow through the baffle plates, the mixture is divided and then recombined to distribute the gas within the adhesive.
Although the device disclosed in the '529 patent performs satisfactorily when used in conjunction with many liquids, and specifically hot melt adhesives having much lower viscosities than the materials used in, for example, the dynamic mixer of the '631 patent mention above, testing has shown that higher viscosity liquid polymers having viscosities above about 3,000 cps cannot be formed into high quality foams with the dispensing head of the '529 patent. More specifically, when plastisols having viscosities on the order of 3,000 cps and above are run through the dispensing head disclosed in the '529 patent, inadequate dispersion of the gas within the plastisol results in low quality foam which is unsuitable for many applications. Thus, the dispensing head of the '529 patent is not suited for producing the high quality foamed polymeric material which is produced by the dynamic mixer of the '631 patent mentioned above.
In addition to the problems associated with attempting to produce high quality foam from high viscosity materials using apparatus of the type shown in the '529 patent, uneven gas distribution may result in certain liquids having even lower viscosities, e.g., in the range of about 2000-3000 cps. It is theorized that problems associated with the use of such lower viscosity materials may relate more to an inability of the material to hold gas in a homogeneous dispersion than to the viscosity of the material.
The '712 patent to Cobbs, Jr. et al. relates to methods and apparatus for dispensing foams or atomizable paints. More particularly, the '712 patent utilizes a combined heater and mixer to coincidentally heat and mix a gas and a liquid polymer. The heater/mixer is contained in a circulating loop which further includes a feed pump for force feeding liquid polymer into a metering block disposed at the inlet to the heater/mixer. The circulating loop also includes one or more dispensing guns and a recirculation pump for maintaining the system pressure in the loop. The metering block further receives a blowing agent or gas which is mixed with the liquid polymer in the heater/mixer. The heater/mixer is disclosed as including either a static mixer or a dynamic mixer. With specific regard to the static mixer alternative, a conventional static mixer formed with 21 elements of right and left-handed helices is disclosed.
Several disadvantages are associated with a recirculation type system with regard to the production of foams with highly viscous liquid polymers. Namely, the use of a circulation loop which necessitates both a feed pump and a recirculation pump increases the size and cost of the system as compared to systems which do not require such a loop. Moreover, the degree of mixing is strictly dependent on the ratio between the difference in flow rates through the heater/mixer and the despensing gun or guns. In other words, to obtain better mixing the flow rate through the heater/mixer must be much higher than the flow rate through the dispensing gun or guns. Therefore, there must be a relatively high flow rate through the heater/mixer to achieve adequate mixing, however, the comparably low flow rate through the dispensing gun or guns causes the system to be inefficient and unusable in many applications. This is especially true in the case of highly viscous polymers which do not easily achieve high flow rates.
In view of the above noted problems in the prior art, there is a need for improvements in the formation of high quality closed cell foam from higher viscosity polymers, and specifically for improvements which increase the efficiency and cost effectiveness of producing such high quality closed cell foam.